The relief of pain long has been, and remains, one of the primary aims of medicine. Unfortunately, almost all potent analgetic agents produce at least some degree of reaction other than the desired analgesia, particularly when taken in large doses for moderate to severe pain. Such side effects include dizziness, blurring or dimness of vision, nausea, vomiting, increased or depressed respiratory or pulse rates, and the like. Accordingly, in the use of analgetic agents, there must be taken into account not only the analgetic potency of the particular agent but also its potential side-effect liability. It is known to prescribe mixtures of analgetics even though their activites are merely additive, since side-effects may be reduced by combining analgetics which act by different mechanisms. It is also known that, in some instances, mixtures of two or more analgetic agents potentiate each other's activities. This is particularly desirable since the total amount of drug can be reduced and the side-effects decreased. Thus, there is a continuing search for combinations of analgetic agents which produce an enhanced (synergistic) analgetic effect, in an attempt to find particular combinations which produce maximum analgesia with little or no side-effects.